Recently, it is required for a semiconductor device to increase withstand voltage and to reduce on-state resistance. Therefore, a SJ (super junction) structure type semiconductor device is well-known to have high withstand voltage and low on-state resistance. The SJ structure is formed in a drift layer of the device. The drift layer includes an N conductive type column (N column) and a P conductive type column (P column). The N column and the P column compose a unit as a pair of the SJ structure so that multiple pairs of the N column and the P column provide the SJ structure. The N column includes an N conductive type impurity, and the P column includes a P conductive type impurity. Further, the device is composed of a center region and a periphery region. In the center region, a semiconductor switching device is formed. In the periphery region, no semiconductor switching device is formed. Here, the drift layer is disposed from the center region to the periphery region.
When the device turns off, the N column and the P column in the SJ structure are depleted, respectively, since a depletion layer extending from each P-N junction boundary between the N column and the P column. Thus, the device has high withstand voltage. To deplete the N column and the P column completely and substantially, it is required to equalize impurity amounts of the N column and the P column. Specifically, the impurity amount of the N column is controlled to be equal to the impurity amount of the P column so that both columns are completely depleted. However, in the periphery region, when the impurity amounts of the N column and the P column are equalized, sufficient withstand voltage of the device is not obtained. This is because formation of the depletion layer in the periphery region is different from that in the center region, since a contact region for fixing electric potential of the P column is not formed in the periphery region. Here, in general, the electric potential of the P column is fixed to null, i.e., ground potential. In the periphery region, when the impurity amount of the N column is equalized to that of the P column, the N column and the P column in the periphery region are not sufficiently depleted, compared with the center region. Specifically, the depletion layer of the periphery region is smaller than that of the center region. Accordingly, the total withstand voltage of the device is limited to the withstand voltage of the periphery region. Thus, the withstand voltage of the device is reduced.
To improve the above problem, a semiconductor device with a periphery region and a center region, which have different impurity amounts, is disclosed in U.S. Pat. No. 6,844,592. Specifically, the P column in the periphery region has an excess impurity amount. In this case, the withstand voltage of the periphery region is improved so that the withstand voltage of the periphery region is equal to that of the center portion. Thus, the total withstand voltage of the device is improved.
The inventors have further studied about a SJ type semiconductor device. As a result, the inventors found new knowledge as follows. When the N column and the P column in the SJ structure are formed, the impurity amount of each column may be deviated. Specifically, the deviation of the impurity amount is caused by deviation of the impurity concentration and width of each column. The deviation of the impurity amount affects the withstand voltage of the device so that the withstand voltage is extremely reduced. This effect of the deviation is caused in the periphery region in particular. Thus, when the deviation of the impurity amount in the manufacturing process of the device deteriorates the withstand voltage, yielding ratio of product in the manufacturing process is reduced so that the total manufacturing cost increases.